The present invention relates to refining elements for use in the refining of various fibrous materials, such as cellulosic and lignocellulosic materials, including wood chips, raw or pretreated, and wood pulp, and particularly to a method of manufacturing a refining element with an abrasive comminuting surface and the refining element produced thereby.
Various fibrous materials, such as wood chips, whether raw or pretreated with steam and/or chemicals, are commonly mechanically refined, i.e., defibered, in an apparatus known as a rotating disc refiner. In such devices, the fibrous material is defibered or refined by mechanical action during its passage through a narrow gap between two closely spaced opposed working surfaces. These working surfaces generally comprise annular refining plates formed of a plurality of truncated circular sector shaped elements arranged in circumferentially adjacent relationship to form an annular comminuting surface. Due to rotation of one or both of the working surfaces, the fibrous material is defibered by mechanical action as it passes outwardly from the inner radius of the refiner plates to the outer radius of the refiner plates under the forces of rotation.
A typical refining plate useful in such disc refiners for refining fibrous materials, in particular wood chips, is formed of a plurality of truncated circular sector-shaped elements disposed in circumferentially adjacent relationship to form an annular comminuting surface. The comminuting surface of the face of each refining element is divided by one or more circular arcs into a plurality of refining regions, typically two or three. The first region, comprising the radially inwardmost region, is provided with a series of substantially radially directed breaker bars forming a series of relatively widely spaced ridges and grooves. The second region, which lies radially outward of the first region and adjacent thereto, is provided with a series of somewhat thinner substantially radially extending bars forming a series of narrower more closely spaced ridges and grooves. If there is a third region, it lies radially outward of and adjacent to the second region, i.e., the intermediate region, and it is provided with even thinner substantially radially extending bars forming a series of still narrower and even more closely spaced ridges and grooves.
Another type of refining element useful in such disc refiners for refining fibrous materials, in particular wood chips, is presented in U.S. Pat. No. 4,372,495. As disclosed therein, the refining plate is formed of a plurality of truncated circular sector-shaped elements disposed in circumferentially adjacent relationship to form an annular comminuting surface. The comminuting surface on the face of each refining element is divided by an arc of a circle into adjacent first and second regions, the first region being radially inward of the second region, with the first region being provided with a series of breaker bars forming a series of relatively widely spaced ridges and grooves, and the second region lying radially outward of the first region and being provided with an abrasive disintegrating surface formed by abrasive particles having an average grit size of between about 12 and 120 grit, i.e., about 140 micrometers to 0.25 centimeters. The abrasive material may be a ceramic material, such as silica, alumina, silicon carbide, zirconia and tungsten carbide.
The abrasive surface of the refining elements disclosed in U.S. Pat. No. 4,372,495 is formed by brazing ceramic particles, generally tungsten carbide grit of 36 grit, to the surface of the stainless steel element. The process is carried out by applying a layer of brazing powder, typically having a nickel-chromium-boron matrix, to the stainless steel substrate of the refiner element. After the brazing powder is applied to a thickness of 0.010 to 0.015 inches, the tungsten carbide powder is applied in a single layer over the brazing powder layer. The tungsten carbide powder is then wetted with a fluoride based flux. This layering process is repeated several times until an overall coating thickness of 0.090 inches is obtained. After the coating mixture is dried, the refining element is placed in a vacuum furnace which is brought up to brazing temperature over an eight hour period. The refining element is held at a brazing temperature of 2050.degree. F. for a period of one hour. The refining element is then allowed to cool in the vacuum furnace for one hour to a temperature below 1OOO.degree. F., after which the brazed refiner plate is removed and allowed to cool overnight.
In addition to being very time consuming, labor intensive, this brazing process produces an abrasive layer which is subject to flaking of the abrasive layer from the metal substrate due to built-in stress that results in the coating bond as the brazed element cools as a result of the difference in the coefficients of thermal expansion of the tungsten carbide grit and the stainless steel substrate.
Another problem is warping of the refiner elements during the brazing process. Often the refiner elements must be placed in the furnace for a second brazing to achieve a strong bond or to repair an incomplete or flaked coating. Further, the effectiveness of the abrasive coating is reduced as the abrasive grit is glazed over during the brazing process thereby dulling the sharp edges of the grit.
It is an object of the present invention to provide a method of manufacturing a refining element having an abrasive comminuting surface without brazing the abrasive particles to the metallic substrate of the refiner elements thereby avoiding subjecting the refiner elements to high furnace temperatures.
It is a further object of the present invention to provide a refiner element, and a refiner plate formed of a plurality of such refiner elements, having at least a region of its comminuting surface comprising an abrasive surface formed by abrasive particles implanted into, rather than brazed onto, the metallic substrate of the refiner element.